Plague -- United States, 1992

From January 1 through October 15, 1992, 11 human plague
cases, including one described recently (1), have been reported in
the United States. This report summarizes the epidemiologic
information on these cases and provides recommendations for control
of plague.

Three of the 11 cases occurred among young children (aged less
than or equal to 6 years); eight occurred among adolescents and
adults (Table 1). Ten cases occurred among males. Four of the cases
occurred among American Indians (three among Navajo and one in a
Paiute Indian). Four cases were reported from Arizona, three from
New Mexico, and one each from California, Idaho, Nevada, and Utah.
Fleas were implicated as the source of infection in seven cases,
domestic cats in two, and a wild rodent carcass in one; the source
of one case was undetermined. In six cases, the infective exposure
probably occurred near the patient's residence. All but one patient
recovered with antibiotic treatment.

Editorial Note

Editorial Note: From 1950 through 1991, 336 cases of plague were
reported in the United States (Figure 1) (2; CDC, unpublished data,
1991); more than half (194 [58%]) of these have occurred since
1980. Yersinia pestis, the etiologic agent of plague, is maintained
in the western United States as an enzootic infection of rodents
and their fleas (3,4). Humans become infected through contact with
infected animals or fleas, usually during epizootics among rodent
species (3). Although human plague cases were reported in 13
western states during 1950-1991, Arizona (14%), California (10%),
Colorado (10%), and New Mexico (56%) reported 302 (90%) of the
cases, and 97 (29%) were among American Indians. The Navajo
composed 77 (23%) of the total cases and 79% of the cases among
American Indians (5). The Paiute case-patient is the first reported
among this tribe and the first human plague case reported in
northeastern Nevada/southwestern Idaho.

American Indians are disproportionately represented among
plague cases in the United States, possibly because they frequently
reside in rural areas where plague is enzootic among rodents.
Because prairie dogs (Cynomys spp.) are abundant and extremely
susceptible to plague, they are a major amplifying host throughout
the Southwest, including the Navajo Nation (3-5). Epizootics also
frequently involve rock squirrels (Spermophilus variegatus),
usually in conjunction with prairie dog epizootics. Rock squirrels
have a predilection for habitats created by human activity and
often live in peridomestic sites (e.g., abandoned cars, rock walls,
and mobile-home foundations). Other reasons for the high proportion
of plague cases among the Navajo include traditional involvement in
pastoral activities such as sheep-herding, which place Navajos in
close contact with rodent habitat; killing of prairie dogs for
consumption by humans and by domestic animals; and ownership of
free-roaming dogs and cats.

The cyclical pattern of plague cases in humans probably
reflects the secular trend of plague epizootics in rodents (3).
Epizootics among ground squirrels and prairie dogs cause major
reductions in the populations of these animals, such as the
depopulation following the large southwestern epizootic that
occurred during 1983-1985 (CDC, unpublished data, 1991). The recent
recovery of these rodent populations (CDC, unpublished data, 1991)
suggests that increased risk for human cases will persist into 1993
and beyond.

Plague surveillance and control in enzootic areas should
include 1) monitoring die-offs in rodent populations using the
public to report sightings of dead animals and reductions in colony
size; 2) educating the medical and veterinary communities
concerning the manifestations and diagnosis of plague -- including
the pneumonic syndrome (1) -- and procedures for reporting
suspected
cases; 3) systematic monitoring of rodent plague activity in
populated areas by public health personnel, and controlling fleas
in areas of rodent epizootics near places of human activity; and 4)
educating the public about the role of domestic animals, such as
cats and dogs, in zoonotic transmission of the disease, including
information about control of fleas on pets and confinement of pets.

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